In semiconductor devices, there are occasions where it is necessary to generate an internal supply voltage higher than a voltage externally supplied from an external power supply. For example, a phase-change memory, which is a non-volatile semiconductor storage device and one type of variable-resistance memory cell through which a current is passed to store information, requires a high voltage and much current to write data. For this reason, the semiconductor device is provided internally with a charge pump circuit, the externally provided supply voltage is boosted and the boosted voltage is used in a write circuit. In a phase-change memory, a set pulse or a reset pulse is applied to phase-change elements to thereby pass the corresponding currents, and each of the phase-change elements is set to an amorphous state or to a crystalline state by the temperature profile of the heat produced, whereby each phase-change element stores the corresponding data. Placing a phase-change element in the amorphous state rather than the crystalline state requires the passage of a larger current (a major portion of the current) in a shorter time. On the other hand, placing a phase-change element in the crystalline state rather than the amorphous state requires passing current over a longer period of time.
FIG. 6 of Patent Document 1 illustrates a charge pump circuit for supplying a write (program) pulse generator of a phase-change memory with power.
Patent Document 2 describes a phase-change memory using a normally operating auxiliary pump and a main pump, which operates only when the phase-change memory is programmed, thereby hastening start-up of the programming operation in comparison with the prior art in which charge pumping is started after programming commences.
Patent Document 3 does not relate to a phase-change device but describes a switching power supply device in which a plurality of charge pump circuits are serially connected, wherein the serially connected charge pump circuits are controlled synchronously in order to reduce ripple noise in the output voltage.
[Patent Documents 1]
    Japanese Patent Kokai Publication No. JP-P2008-165964A, which corresponds to US 2008/137401A1.[Patent Documents 2]    Japanese Patent Kokai Publication No. JP-P2007-087568A, which corresponds to US 2007/064473A1.[Patent Documents 3]    Japanese Patent Kokai Publication No. JP-P2007-074797A, which corresponds to US 2007/091655A1.[Patent Document 4]    US 2008/180163A1